1. Field of the Invention
The present invention relates to the production of anodic oxide films on aluminium (including aluminium alloys), which are perceived by the viewer as coloured due to optical interference effects within the film.
2. Prior Art
The colouring of anodic oxide films by electrolytic deposition of inorganic particles has become well known. In the electrocolouring process inorganic material is deposited in the pores of the anodic oxide film by the passage of electric current, usually alternating current, between an anodised aluminium surface and a counterelectrode, whilst immersed in an acidic bath of an appropriate metal salt. The most commonly employed electrolytes are salts of nickel, cobalt, tin and copper. The counterelectrode is usually graphite or stainless steel, although nickel, tin and copper electrodes are also employed when the bath contains the salt of the corresponding metal. The deposits of material constitute what are referred to herein as inorganic pigmentary deposits, although the mechanism by which they function to give a coloured appearance is quite different from that of normal organic or inorganic pigments.
In a conventional electrocolouring process, employing, for example, a nickel sulphate electrolyte, the colours obtained range from golden brown through dark bronze to black with increase in treatment time and applied voltage. It is believed that in the conventional coloured anodic oxide coatings the dark colours are the result of the scattering and absorption within the coating of the light reflected from the surface of the underlying aluminium metal. The gold to bronze colours are believed to be due to greater absorption of the shorter wave length light, i.e. in the blue-violet range. As the pores of the oxide film become increasingly filled with pigmentary deposits the extent of the absorption of light within the film becomes almost total, so that the film acquires an almost completely black appearance.
In current commercial practice anodising in a sulphuric acid-based electrolyte has almost totally replaced all other anodising processes for the production of thick, clear, porous-type anodic oxide coatings. In general, direct current voltages in the range of 12-22 volts are used in sulphuric acid-based electrolytes depending upon the strength and temperature of the acid. Sulphuric acid-based electrolytes include mixtures of sulphuric acid with other acids, such as oxalic acid and sulphamic acid, in which the anodising characteristics are broadly determined by the sulphuric acid content. Typically in sulphuric acid anodising the electrolyte contains 15-20% (by weight) sulphuric acid at a temperature of 20.degree. C. and a voltage of 17-18 volts is employed.
It has been shown (G. C. Wood and J. P. O'Sullivan: Electrochimica Acta 15 1865-76 (1970) that in a porous-type anodic aluminium oxide film the pores are at essentially uniform spacing so that each pore may be considered as the centre of an essentially hexagonal cell. There is a barrier layer of aluminium oxide between the bottom of the pore and the surface of the metal. The pore diameter, cell size and barrier layer thickness each have a virtually linear relationship with the applied anodising voltage. This relationship holds true within quite small deviations for other electrolytes employed in anodising aluminium, for example chromic acid and oxidic acid.
In normal sulphuric acid anodising, the pore diameter is in the range of 150-180 A (15-18 nm) and the applied voltage is 17-18 volts. The barrier layer thickness is about equal to the pore diameter and the cell size is about 400-500 A (40-50 nm). The same holds true with mixed sulphuric acid-oxalic acid electrolytes.
U.S. Pat. No. 4,066,816 describes a process in which a new range of colours was obtained by electrocolouring, the apparent colour being due to optical interference in addition to the scattering and absorption effects already noted. In order to obtain these optical interference colours it was found necessary to increase the cross section of the pores of a sulphuric acid-anodised film in a region adjacent the aluminium oxide/aluminium interface so as to increase the size of the outer ends of the pigmentary deposits. Without that essential step it was found impossible to obtain satisfactory colours by light interference effects in such film.
Since the perceived colour is the result of interference between light scattered from the outer ends of the individual deposits and light scattered from the aluminium/aluminium oxide interface the outer ends of the individual deposits must be of adequate size and when reference is made to the size of the deposits, it is the size of the outer ends which is relevant in this context. The colour produced depends on the difference in optical path resulting from separation of the two light scattering surfaces (the outer ends of the deposits and the aluminium oxide/aluminium interface). To produce perceived colours by interference this separation should be at least 50 nm; as stated in the aforementioned U.S. Pat. No. 4,066,816, this separation should be in the range of 500-3000 A (50-300 nm). The separation, when colouring a particular film, depended on the height of the deposits. It was found that a range of attractive colours, including blue-grey, yellow-green, orange-brown and purple, can be produced by electrolytic colouring when employing interference colouring effects.
In Journal of Japanese Anodising Association (Aluminium Kenkyu Kaisha) 1976 No. 2 pp 17-18, I. Inoue and others have described a modification of the conventional electrocolouring process devised by Asada in which an anodic oxide film is produced by anodising in sulphuric acid under D.C. followed by electrocolouring under A.C. conditions. Inoue and his co-workers found that blue, violet and green shades could be obtained by adding phosphoric acid to the ordinary colouring electrolyte. They found that they could also obtain the same colours by treating the D.C.-anodised material in phosphoric acid under A.C. conditions before carrying out the electrocolouring treatment. They further found that these colours could be obtained by carrying out the anodising at low A.C. voltage in phosphoric acid in place of conventional D.C. anodising in sulphuric acid before the electrocolouring step. The authors concluded that the colours were due to geometrical change of the barrier layer as a result of the addition of phosphoric acid with resultant dispersed deposition of metals in the electrocolouring stage giving rise to blue, violet or green colouring of the film.
In U.S. Pat. No. 4,066,816 one described route for increasing the size of the relevant portion of the pores was the passage of direct current between the aluminium article, connected as anode, and a counter electrode in a phosphoric acid-based electrolyte at voltages in the range of 8-50 volts.